The present invention relates to suspension assemblies for microactuators In particular, the present invention relates to suspension assemblies for air bearing sliders which have a microactuator positioning the air bearing slider relative to a suspension arm in a disc drive.
Magnetic discs are commonly used to store information in computer devices. Each side of the disc has its own associated magnetic head assembly used for reading and writing information thereon. To record information on the disc, the write head creates a highly concentrated magnetic field. During writing, the strength of the concentrated magnetic field directly under the write head is greater than the coercivity of the recording medium (known as "saturating" the medium), and grains of the recording medium at that location are magnetized with a direction which matches the direction of the applied magnetic field. The grains of the recording medium retain their magnetization after the saturating magnetic field is removed. As the disc rotates, the direction of the writing magnetic field is alternated based on bits of the information being stored, thereby recording a magnetic pattern on the track directly under the write head.
Each magnetic head assembly is mounted on the end of a support or actuator arm, which positions the head directly adjacent the disc surface. If the actuator arm is held stationary, the magnetic head assembly will pass over a generally circular path on the disc known as a track, and information can be read from or written to that track. Each concentric track has a unique radius, and reading and writing information from or to a specific track requires the magnetic head to be located above the track. By moving the actuator arm, the magnetic head assembly is moved radially on the disc surface between tracks.
Each magnetic head assembly is typically connected to its respective actuator arm by a flexure or "suspension" arm. The suspension arm functions as a bending spring to bias the magnetic head assembly toward the disc surface. For instance, the suspension arm may bias the magnetic head assembly toward the disc with a preload force of 4 grams in a "vertical" direction.
The magnetic head assembly includes a portion known as a "slider". As the disc pack rotates at high speed (typically in excess of 10 m/s relative to the slider), the aerodynamic properties of the slider cause the magnetic head assembly to "fly" above its respective disc surface. When the disc drive is turned off, the slider containing the read/write transducer lands and rests on the disc until the disc drive is again started up.
At all times, the suspension arm applies the vertical preload force to the slider. When the power is switched on in a disc drive, the suspension arm applies a longitudinal force to the slider to overcome the static friction or "stiction" force on the slider. During use, the suspension arm applies longitudinal force to the slider responsive to the drag force associated with the slider's dynamic friction contact with the air and/or disc. The suspension arm may also apply pitch, roll and yaw forces to keep the slider at a desired attitude relative to the disc surface.
The computer industry continually seeks to reduce the size of computer components and to increase the speed at which computer components operate. To this end, it is desired to reduce the size required to magnetically record bits of information. It is concomitantly important to maintain the integrity of the information as size is decreased, and magnetic storage of information must be virtually 100% error free.
To reduce size and increase storage capacity in magnetic disc drive units, magnetic discs have been stacked in a "pack" all carried within the same disc drive unit. All of the discs are connected together on a single spindle and rotate about the same axis. To further increase space savings in the disc drive unit, all of the actuator arms are connected together and pivot about the same pivot point. A single servomotor typically controls all of the actuator arms and their associated magnetic heads. This configuration is identified as an "E-block", which refers to the "E" type of shape formed by the adjacent arms and the servomotor/pivot assembly. To further increase space savings in the disc drive unit, the thickness of discs and the axial spacing between discs has been compressed.
Several parameters of the disc drive system are critical for increasing storage density for a given area of disc space. Higher coercivity in the magnetic media and smaller head to media spacing lead to smaller transition size and increased storage density. Smaller width or minimum separation between adjacent tracks on the disc also increases storage density. However, each track must be readable by the read head without interference or cross-talk from adjacent tracks. Each track must also be able retain its recorded information without alteration during writing of adjacent tracks. Present track spacing of commercially available discs is in the range of 5,000 to 10,000 tracks per radial inch, e.g., each track has a width of about 2.5 to 5 microns (25,000 to 50,000 Angstroms).
The disc drive must be able to differentiate between tracks on the disc and to center the magnetic head over any particular track. Most disc drives use embedded "servo patterns" of recorded information on the disc. The servo patterns are read by the magnetic head assembly to inform the disc drive of track location. "Off track errors" result from a number of factors inherent in the disc drive system. The major source of off track error is the spindle motor bearings. Self-induced vibration of the spindle is caused by the rotating disc stack, and gets worse with increasing number of discs on the stack Actuator arm bearings introduce error, and flex cables may exert bias forces on the slider. Residual vibration also contributes to off track error. There are other non-mechanical sources of off track errors as well, such as defects in the media, non-linearity in the several patterns, etc. In addition to internal sources, one other major source of off track error is vibration and shock from outside the drives, such as from an unbalanced cooling fan of the PC. With these off track errors, servo patterns are also used to continually correct to a centered position over the desired track.
As track width decreases, it becomes more and more important for the magnetic head to be consistently and accurately positioned over the track. One method to increase the positioning accuracy of the slider is through a microactuator positioned between the suspension arm and the slider. The microactuator "piggy backs" in dual stage performance in addition to the macro positioning provided by the actuator arm. The microactuator allows fine level tracking of each individual transducer without movement of the actuator arm, and allows an increase in the correction frequency.
The ideal microactuator for magnetic disc drives should have minimal height or thickness (to preserve disc to disc packaging advantage) minimal width (to maximize useable real estate), minimal mass (to maintain high access speed during movement of the actuator arm), and minimal added cost.
Proposed microactuators include a pad for attachment to the suspension arm and a pad for attachment to the slider separated by some sort of microspring structure. The suspension arm pad and the slider arm pad are then moved relative to each other by a small electromagnet or micro-coil. To meet the desired size and heating constraints, the micro-coil may be capable of producing a force of on the order of 100 .mu.N for continuous duty and up to about 1 mN for short pulses, across a 10.mu.m gap. The force produced is inversely proportional to the square of the gap, so a smaller gap will permit larger forces for a given maximum current. The structure of the microactuator is further discussed in Application No. 169-12.340 entitled MAGNETIC MICROACTUATOR AND INDUCTIVE SENSOR HAVING SHAPED POLE CONFIGURATION, assigned to the assignee of the present invention and incorporated herein by reference.
Depending on the microbeam or microspring structure, the microactuator may be rotary or lateral A rotary microactuator rotates or pivots the slider about a central vertical slider axis to finely position the transducer. A lateral microactuator moves the slider laterally to finely position the transducer. Rotary motion has some potential advantages including faster possible positioning for the same force input, if the point of rotation is near the center of mass of the slider, and also reduced sensitivity to disturbances generated by the primary servo or voice coil motion. However, in part as a consequence of the desire to keep the entire structure narrow, convenient spring designs tend to produce linear rather than rotary motion.
The magnitude of the displacement force in the lateral direction should be minimal to permit sufficient flexibility in the direction of desired motion to correct the lateral position of the slider. At the same time, the microbeam or microspring structure must be stiff enough in the vertical and longitudinal directions to withstand and transit the necessary preload, drag, pitch, roll and yaw forces on the slider with minimal vibration.
The suspension arm pad, the slider arm pad and the microbeam or microspring structure may all be integrally formed of single crystal silicon or similar structure. The microbeam or microspring structure needs to provide a consistent displacement force so the microcoil can accurately adjust the position of the slider. Any fracture of the single crystal silicon material, caused typically by surface defects, greatly affects the displacement force and is a significant, possibly fatal, problem.